Measurements of fluence profiles in femtosecond laser filaments in air

We introduce a technique to measure fluence distributions in femtosecond laser beams with peak intensity of up to several hundred terawatts per square centimeter. Our ap- proach is based on the dependence of single-shot laser abla- tion threshold for gold on the angle of incidence of the laser beam on the gold sample. We apply this technique to the profiling of fluence distributions in femtosecond laser fila- ments at a wavelength of 800 nm in air. The peak intensity is found to be clamped at a level that depends on the ex- ternal beam focusing. The limiting value of the peak inten- sity attainable in long-range 800 nm air filaments, under very loose focusing conditions (f -number above ∼500), is about 55 TW∕cm2.